The present inventions relate to improving the quality of products produced by plastic resin extrusion lines.
When film is extruded, it typically is in the form of a flat continuous web as in cast film extrusion or a tubular form as in blown film extrusion. In blown film, the inflated tubular, bubble-type form passes through stabilizers of various designs and into a flattening device. This device, known as a collapsing frame, flattens the tube into a two sided, connected film with no air inside. In a flat state, webs are conveyed by various combinations of non-motorized and motorized rollers to a winder or to in-line downstream conversion, equipment such as printing presses, laminators, or bag machines. In a winder, a web may be cut into individual webs before the winding process, and the resulting rolls are converted at a later time in what is commonly termed an out-of-line converting process.
As film webs are conveyed to a winder or in-line conversion equipment, motorized rollers can be used at various points to maintain control of web tension. Except for the initial motorized roll immediately downstream of the extrusion process (commonly called the primary nip roll) where constant motorized roll speed is maintained without feedback, tension is typically measured in some way and provided as feedback to a motor controller. This motor controller is programmed to respond to maintain tension at a constant preset setting.
Typically, tension is measured either statically by measuring the applied force due to tension on load cell devices attached to idler rolls, or somewhat dynamically through what is commonly referred to as a dancer. A dancer is a series of idler rolls that move against springs, counterweights, air cylinders, or other such force applying devices in such a way as to allow the film's path length to change in response to tension variations and thus provide indication of film tension. In other cases, tension is measured indirectly by measuring the torque applied by the motorized roll and comparing the measurement to a no-load torque pre-measured in the absence of the web, taking into consideration roll geometry, and converting this to applied web tension.
An issue with maintaining proper tension control is web slippage when passing over motorized rolls. In many cases, additional rolls are pressed against the motorized roll to form a nip point that the web passes. The nip point acts to help hold the film against the motorized roll to prevent slippage. In other cases, large wrap angles around the motorized roll or more than one roll are used to provide a large enough surface area for friction to act and prevent slippage. Combinations of nipping and large wrap angle may not always prevent slippage and thus can lead to tension control problems as the web becomes uncontrollable.
One such area is the motorized roll used in the winding process known commonly as the lay-on or winding drum. The winding drum is the final motorized roll the web passes over before being wound on the finished roll. Good tension control should be maintained by the winding drum or undesirable defects in the roll can result. In some winding machines, the finished roll shaft is also motorized to aid in maintaining proper tension control of the web as the finished roll builds in diameter to a final roll diameter over time such as disclosed in U.S. Pat. No. 5,275,348.
In practice it is desirable to maintain constant tension before reaching the winder drum for such purposes as slitting the web into multiple webs, slitting trim from the edges or middle for maintaining high quality roll ends, and minimizing wrinkling and deformation of the web before winding on the finished roll. It is further desirable to control the tension on the finished roll to tensions that are different from upstream tension especially when making very large rolls. Excessive winding tension can create roll quality issues or even crush the central winding core. However, higher tensions are beneficial to the upstream cutting and trimming processes.
A limitation of some devices, such as the winding devices described in U.S. Pat. No. 5,275,348, is that they require essentially zero winding drum for proper tension control to be achieved. In practice, devices such as these exhibit slippage when significant differences exist in tension upstream and downstream of the motorized drum roll and thus limit the tension difference achievable. In practice, depending on web and drum materials used, small differences of as little as 0.1 or 0.2 pounds per linear inch of web width across the face of the motorized roll are enough to cause slippage of the web. Typical web tensions within these extrusion processes range from 0.25 to 2.0 pounds per linear inch.
A situation for maximum tension differential exists when winders, which are required to continuously handle a web without interruption, are transitioning from winding one roll to the next. Extrusion processes are run continuously, so the web is severed at the conclusion of building a finished roll to final diameter and the loose incoming end is taken up onto a new winding core to start building a new roll without stopping or slowing the upstream process. The severing process causes tension in the web local to the severing device to suddenly drop to zero, creating instantaneous slippage on the motorized drum roll since now tension differences are maximized.
U.S. Pat. No. 5,848,761 shows one example of a device in which a severing knife is contained within a motorized winding drum. This arrangement makes the slippage problem especially troublesome. In this case, a vacuum chamber within the drum roll is energized to hold the web in place and prevent slippage until a new winding core acquires a loose incoming cut edge of the web and reestablishes web tension. In practice, some slippage does still occur and the complexity of such systems is very costly.
Prior methods exist for acquiring a loose incoming cut edge of a web. Sticky substances such as glue or tape are popular but messy and typically create undesirable impressions in the wound web. Other techniques as shown in U.S. Pat. No. 4,852,820 employ an electrostatic charging device between a motorized winding drum and a finished roll that is about to be cut free. This eliminates the problems associated with using glue or tape. The incoming winding core is held generally opposite the electrostatic charging device after the motorized winding drum with the web passing in between. Just before severing the web, an electrostatic charge is applied to the web opposite the incoming winding core. This causes an electrostatic force that acts to push the web toward the incoming winding core. This force of attraction is due to electrostatically formed ions preferably placed on the opposite side of the web. These ions are drawn to the ground potential of the incoming conductive and grounded winding shaft which holds the incoming typically not conductive, winding core. These ions act to carry the web with them and try to attach the web to the winding core. The web is then severed nearby the charging device and the completed finished roll. The loose incoming web end is electrostatically attracted to the incoming winding shaft where it attaches to the new core and begins to build a new roll.
One drawback of motorized drum rolls as shown in U.S. Pat. No. 5,848,761 containing the severing knife within itself is that the elect positioned between the severing knife and the motorized winding drum because these latter two are formed in one unit. This makes it impossible to use the electrostatic transfer method described in U.S. Pat. No. 4,852,820.